1. Technical Field of the Invention
This invention relates to a shaped aluminum article. More particularly, this invention relates to an aluminum article exhibiting high strength and good corrosion resistance produced from a zinc-magnesium type aluminum base alloy.
2. Description of the Prior Art
In recent years, due to increased interest in conservation and recycling of materials, new uses for aluminum have been proposed due to its light weight and reusability. This has become of particular interest in applications, such as the automobile industry where weight reduction can result in fuel conservation. It is desirable that the aluminum replacement, however, be able to match the strength and corrosion resistance of the metal which it is replacing. It is also desirable that the fabrication of the part should be economically competitive with current fabricating techniques used on the materials being replaced.
For example, aluminum has been used as a replacement for steel in automobile bumpers. This requires the use of an alloy which can provide sufficient strength to resist at least the minimum impact of another vehicle, meet corrosion resistance requirements, including both resistance to exfoliation and stress-corrosion cracking, and have good forming or fabrication characteristics.
Aluminum Association 7000 series alloys, which usually contain as the principal ingredients zinc, magnesium, and copper, characteristically combine high strength with resistance to corrosion. However, as requirements increase for aluminum as a replacement for other materials, there remains a need for an aluminum base alloy with strength exceeding the strength of present aluminum alloys while maintaining at least equal corrosion resistant characteristics. While other 7000 series alloys have been proposed as replacements for those currently in use, usually an increase in strength is gained at the expense of reduced resistance to exfoliation or an increased quench sensitivity necessitating the use of water quenching which can result in distortion of the shaped aluminum article made from the alloy, e.g., an aluminum bumper for an automobile. cause distortion of the article made from the alloy, e.g., an automobile bumper for an automobile.
Kawatsu U.S. Pat. No. 3,531,337 discloses an aluminum base alloy containing, as principal additives, 2.6 wt. % zinc and 0.6 to 3.8 wt. % magnesium. In addition to other well known alloying components such as iron, manganese, chromium, zirconium, and silicon, the patentee adds from 0.2 to 1.2 wt. % nickel and/or cobalt and then subjects the alloy to a nitriding process to work harden the product. The addition of at least one of these two metals, together with iron, is said to concentrate around crystalline grain boundary and strengthen the neighborhood of grain boundary by increasing the dislocation density and promote the work hardening.
However, there remains a need for an aluminum base alloy capable of being formed into a shaped article having a yield strength of at least 414MPa (60 ksi) and having satisfactory resistance to both stress-corrosion cracking and exfoliation. Furthermore, the alloy must have sufficiently low quench sensitivity to permit air quenching after heat treatment so that the article formed from the alloy will not be distorted during quenching.